A HOB

Abstract: A hob comprises a substantially planar member (1) having apertures defining hotplate regions, a glass-ceramic insert (4) being positioned in each aperture above a radiant heating element (7). The use of a moulded glass insert corresponding to a hotplate regionoffers several advantages over prior art hobs conventionally comprising a glass-ceramic sheet formed by a flat glass process which extends over a plurality of hotplate regions.

Description: A Hob The present invention relates to hobs of the type which employ radiant heating elements. It is desirable that the heat output of a hob responds quickly to the setting control This is one reason for the popularity of gas appliances where the change in heat output occurs substantially simultaneously with the change in setting. With regard to electric hobs, this desire for a quick response time has resulted in the gradual trend, initially from solid plate to spiral elements, and more recently to hobs comprising a glass-ceramic top plate, commonly referred to as ceramic hobs, using radiant heating elements, such as tungsten halogen lamps, or inductive heating units. Apart from a quick response time over conventional solid plate and spiral type hobs, ceramic hobs also give the advantage of being easy to clean and are aesthetically pleasing, normally comprising four or more heating units mounted under a single sheet of glass-ceramic. In order to provide at least one hotplate surface in an otherwise conventional solid plate hob with the rapid response time afforded by glass-ceramic hobs, it has been proposed by the present Applicant to produce glass-ceramic "drop-in" units which could be substituted for one or more of the solid plate elements. This is disclosed in UK Patents GB 2177578B and GB 2138935B. According to the present invention there is provided a hob comprising: a substantially planar member having at least one aperture therein defining a hotplate region; a substantially planar glass-ceramic insert located in the aperture and a radiant heating element below the glass-ceramic insert, the glass-ceramic insert being moulded. The present invention retains the advantages provided by a conventional ceramic hob, either incorporating a tungsten halogen or inductive heating source, of fast response time and easy cleaning. However the inventors have realized that by employing a hob in accordance with the present invention significant other advantages are obtained. By using moulded glass-ceramic inserts, a number of features can be incorporated into the hob which would not be possible using glass-ceramic material formed in sheets by the float glass or other process. Also, less glass-ceramic material is used which material is expensive relative to materials used for the member, such as enamelled steel or a suitable works urf ace material. For the avoidance of doubt, if any, the phrase "moulded glass-ceramic insert" includes inserts made by moulding glass which is then heat treated to change the structure of the glass to that of a glass-ceramic. One such material would be lithium-alumino-silicate. One advantage of moulding the insert is that the lower surface of the insert can advantageously be moulded such as to form a lens for modifying the radiation pattern through the insert, to provide a desired heat distribution over the surface of the hotplate region. Advantageously this lens is a Fresnel lens enabling the lowermost surface of the insert to remain substantially planar, thereby avoiding increasing the depth of the insert. In accordance with one embodiment of the invention the insert is moulded to support a temperature sensitive element, substantially the whole of the temperature sensitive region of the element preferably being held in thermal contact of the insert. This enables the heating element to then be controlled in dependence on the temperature glass-ceramic insert instead of on the resistance of the heating element or over temperature cut-out conventionally mounted below the hotplate of a ceramic hob. In an alternative embodiment the insert preferably comprises a temperature sensitive element which in use is connected to a control circuit that regulates the radiant heating element in dependence on the output of the temperature sensitive element. The temperature sensitive element is preferably formed by a region of the glass-ceramic material of the insert, electrodes electrically connected to the control means being formed on the insert defining a region of the insert which is the temperature sensitive element. The glass-ceramic material is a poor electrical conductor, the resistance of which is temperature dependent. By forming electrodes on the insert it is possible to control the heating element directly in dependence upon the resistance and therefore temperature of the glass. The electrodes would normally be closely spaced because of the poor resistive nature of the glass-ceramic, but this ensures the resistive value is not significantly altered by a conductive metal cooking utensil being placed on the upper surface of the insert. The region of the insert forming the temperature sensitive region can have material diffused therein in order to modify the conductive properties of that region. In a further alternative embodiment the temperature sensitive element comprises a temperature sensitive conductive strip deposited on the insert. The electrodes, or conductive strip of the above embodiments are preferably deposited by screen printing, pad printing or by a spray deposition method. The advantage of having a temperature sensitive element directly on the glass ceramic material or formed in that material is that it eliminates the need for both control means for the normal operation of the heating element, often achieved by monitoring the current in the heating elements, and an over temperature protection device located conventionally just below the glass-ceramic layer of a conventional ceramic hob. It is therefore preferable that the control means is electrically connected to the temperature sensitive element and arranged such as to control both the normal heating function of the radiant heating element and also ensure the maximum safe operating temperature of the glass-ceramic insert is not exceeded. Advantageously there is a seal between the insert and the member, which seal is formed from a mixture of alumina/glass fibre and a silicone sealant. Such a mixture prevents the ingress of water or dirt below the insert, and also provides a high resistance to abrasion when the surface is cleaned. Advantageously the hob further comprises a canister mounted below said insert for supporting said heating element, wherein the insert has a downward protruding annular ridge by means of which the canister is located, thereby ensuring that the canister is correctly centred relative to the insert. Preferably the member supports resilient mounting means which urge a canister in which the heating element is supported towards the insert, the canister comprising thermally insulating material which is arranged such that this is urged into contact with the insert. In this way the mass of the thermal insulating material and canister provides damping to any shock force experienced by the glass, but by being mounted to the member itself does not provide an excessive upward force on the insert relative to the insert in response to the member being .depressed. Preferably the member comprises a downwardly protruding annular shelf about the aperture, the shelf supporting the insert and the resilient mounting means such that the attachment of the resilient mounting means to the shelf is obscured from view by the insert. Advantageously the glass-ceramic is thermo-chromic as this provides a visual indication that the glass-ceramic is hot even though the hob may be turned off. According to a second aspect of this invention there is provided a thermo-chromic glass- ceramic material for use in ceramic hobs. One embodiment of the invention will now be described by way of example only with reference to the accompanying figures, in which like reference numerals are used throughout, and of which: Figure 1 is a schematic view of part of a hob in accordance with the present invention, in which those features partially obscured by the ceramic glass insert are shown in broken line; Figure 2 is a cross-section along the line I-I of Figure 1 ; Figures 3 and 4 are alternative configurations of a hob in accordance with the present invention; and Figure 5 is a perspective view of a hob of which figures 1 and 2 or 3 of 4 depict a part. Referring to both Figures 1 and 2, the hob comprises a member 1 of vitreous enamelled steel having an aperture 2 therein. The steel around the edge of the aperture 2 is bent to form a shelf 3 on which is positioned a moulded glass-ceramic insert 4 supported by means of an alumina/glass fibre and silicone mixture. The glass-ceramic inseπ comprises a lithium-alumino-silicate which has a working range in which it can be moulded of 1200-1800°C. Insert 4 has a Fresnel lens 6 moulded onto its lower surface, seen in Figure 2, which controls the dispersion of light from tungsten halogen lamp 7, and on its upper surface spillage retention channel 10. Energisation of the lamp 7 is controlled in dependence on the electrical resistance of the lower surface of insert 4 which is determined by control circuitry (not shown) by measuring the resistive value of the glass between conductors 8a and 8b. These are platinum based electrodes formed by pad printing and processing an appropriate proprietary material, one suitable material being Cermet'™' Platinum Conductor marketed by E.S.L. (Europe). The conductors 8a and 8b are electrically connected at the edge of the insert 4 to control circuitry (not shown) by means of terminal 9. Attached to the lip 3 of the member 1 are three resilient mounting means 11, each comprising a pin and captive spring, which urge upwards metal canister 12, which houses tungsten halogen lamp 7 and additional wire wound heating elements 13. The canister 12 is lined with thermally insulating material 14, a wall portion 15 of which is urged by means of the canister 12 and resilient mounting means 11 into contact with the glass-ceramic insert 4. Further pressure on the glass-ceramic insert 4 by the wall 15 is prevented by the wall 15 coming into contact with the lip 3. Figure 3 shows an alternative arrangement of the hob of Figure 1. In this arrangement the ceramic-glass insert 4 has three downwardly extending legs 16, only one of which is shown. These co-operate with a retention member 17 by which the insert is retained in the canister 12 prior to mounting from underneath the member 1. Figure 4 illustrates an arrangement when the ceramic insert can be inserted into the member 1 from the top, with the canister 12 already in position. A downwardly protruding skirt 18, moulded in the insert 4, retaining the insulating ring 15 such that when inserted the ring 15, in conjunction with the layer of insulating material 14, provides thermally insulated housing for the heating element 7. Figure 5 illustrates a complete hob which Figures 1 and 2 or Figure 3 or Figure 4, depict a part thereof. This hob comprises five hotplates, but it will be realized that any number and shape of hotplates could be employed in a hob in accordance with the invention.

Claims: CLAIMS 1. A hob comprising: a substantially planar member having at least one aperture therein defining a hotplate region; a substantially planar glass-ceramic insert located in the aperture; and a radiant heating element below the glass -ceramic insert, the glass- ceramic insert being moulded. 2. A hob as claimed in claim 1 wherein the lower surface of the insert is moulded such as to form a lens for modifying the radiation pattern through the insert. 3. A hob as claimed in claim 2 wherein the lens is a Fresnel lens. 4. A hob as claimed in any preceding claim wherein the insert is moulded to support a temperature sensitive element. 5. A hob as claimed in claim 4 wherein substantially the whole of a temperature sensitive region of the temperature sensitive element is held in thermal contact with the insert. 6. A hob as claimed in claim 1, 2 or 3 wherein the insert comprises a temperature sensitive element which in use is connected to a control circuit that regulates the radiant heating element in dependence on the output of the temperature sensitive element. 7. A hob as claimed in claim 6 wherein the temperature sensitive element is a temperature sensitive resistive element formed by a region of the glass-ceramic material of the insert. 8. A hob as claimed in claim 7 wherein electrodes electrically connected to the control means are formed on the insert and define the region of the insert which is the temperature sensitive element. 9. A hob as claimed in claim 8 wherein the conduction properties of the region of the insert are modified by diffusion of material into that region. 10. A hob as claimed in claim 1, 2 or 3 wherein the temperature sensitive element comprises a temperature sensitive conductive strip deposited on the insert. 11. A hob as claimed in claim 8, 9 or 10 wherein the electrodes or conductive strip are deposited by screen printing, pad printing or by a spray deposition method. 12. A hob as claimed in any one of claims 4 to 11 wherein control means are electrically connected to the output of the temperature sensitive element and arranged such as to control the normal heating function of the radiant heating element and also ensure the maximum safe operating temperature of the glass-ceramic insert is not exceeded in dependence on the output of the temperature sensitive element. 13. A hob as claimed in any preceding claim further comprising a canister mounted below said insert for supporting said heating element, wherein the insert has a downward protruding annular lip by means of which the canister is located. 14. A hob as claimed in any preceding claim wherein the member supports resilient mounting means which urge a canister, in which the heating element is supported, towards the insert, the canister comprising thermally insulating material which is arranged such that it is urged into contact with the insert. 15. A hob as claimed in claim 14 wherein the member comprises a downwardly protruding annular shelf about the aperture, the shelf supporting the insert and the resilient mounting means. 16. A hob as claimed in any preceding claim wherein the member and insert are arranged such that their upper surfaces are substantially flush. 17. A hob as claimed in any preceding claim further comprising a seal between the insert and the member, which seal is formed from a mixture of alumina/glass fibre and a silicone sealant. 18. A hob as claimed in any preceding claim wherein the glass-ceramic of the insert is thermo-chromic. 19. A hob substantially as hereinbefore described with reference to, or as illustrated in, the accompanying drawings. 20. An insert for use in a hob as claimed in any preceding claim.

Inventors: CARTER MAURICE HUGH, HILL CRAIG ANTHONY, CARTER, MAURICE, HUGH, HILL, CRAIG, ANTHONY
Applicants: CARTER MAURICE HUGH, HILL CRAIG ANTHONY, REDRING ELECTRIC LTD, CARTER, MAURICE, HUGH, HILL, CRAIG, ANTHONY, REDRING ELECTRIC LIMITED
Application year: 1994
Main classification: F24C15
IPC codes: A47J37, F24C7, F24C15, H05B3